This invention relates to the art of method for fabricating thermal heads for thermal recording which are used in various types of printers, plotters, facsimile, recorders and the like as a recording device.
Thermal materials comprising a thermal recording layer on a substrate of a film or the like are commonly used to record images produced in diagnosis by ultrasonic scanning (sonography).
This recording method, also referred to as thermal recording, eliminates the need for wet processing and offers several advantages including convenience in handling. Hence, in recent years, the use of the thermal recording system is not limited to small-scale applications such as diagnosis by ultrasonic scanning and an extension to those areas of medical diagnoses such as CT, MRI and X-ray photography where large and high-quality images are required is under review.
As is well known, thermal recording involves the use of a thermal head having a glaze, in which heating elements are arranged in one direction (main scanning direction) and, with the glaze urged at small pressure against a thermal material, the two members are moved relative to each other in an auxiliary scanning direction perpendicular to the main scanning direction and energy is applied to the respective heating elements in accordance with the image to be recorded in order to heat a thermal recording layer of the thermal material, thereby accomplishing image reproduction.
A protective film is formed on the surface of the glaze of the thermal head in order to protect the heating elements and the like. The protective film is usually made of wear-resistant ceramics such as silicon nitride and the like; however, during thermal recording, the surface of the protective film is heated and kept in sliding contact with the thermal material so that the protective film gradually wears and deteriorates upon repeated recording; hence, the ability of the protective film to protect the heating elements is impaired such that the intended image recording is no longer possible (the head has lost its function).
Particularly in the applications such as the aforementioned medical use which require multiple gradation images of high quality, the trend is toward ensuring the desired high image quality by adopting highly rigid substrates such as polyester film and the like and also by increasing the setting values of recording temperature and pressure at which the thermal head is urged against the thermal material. Therefore, load to be put on the protective film, of the thermal head is large, and wear and corrosion of the protective film are made more likely to progress.
With a view to preventing the wear of the protective film or the thermal head and improving its durability, a number of techniques to improve the performance of the protective film have been considered. Among others, a carbon protective film with carbon as a main component, that is, a carbon-based protective film (hereinafter referred to as carbon protective layer) is known as a protective film excellent in resistance to wear and corrosion.
For example, Examined Published Japanese Patent Application (KOKOKU) No. 61-53955 discloses a thermal head comprising a carbon protective layer having a Vickers hardness of 4500 kg/mm.sup.2 or more as the protective film of the thermal head and, moreover, Unexamined Published Japanese Patent Application (KOKAI) No. 7-132628 discloses a thermal head which has a dual protective film comprising a lower protective layer composed of a silicon-based compound and an upper carbon protective layer (diamond-like carbon layer), whereupon each of such carbon protective layer has properties quite similar to those of diamond including a very high hardness and chemical stability; hence, the carbon protective layer presents sufficiently excellent properties to prevent wear and corrosion which may be caused by the sliding contact with thermal materials.
One of problems in thermal recording employing such a thermal head is in effecting cracking and peeling in the protective film rather easily. Such cracking and peeling give rise to wear and corrosion which results in reduction of durability of the thermal head whereupon the thermal head is not capable of exhibiting high reliability over an extended period of time.
Studies of inventors of the present invention reveal that adhesion between individual layers constituting the protective film and the like play an important role. In other words, if the adhesion between the individual layers constituting the protective film is bad, such cracking and peeling will be effected on the thermal head with repetition of heating and cooling.